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Robertsite spheres (reddish brown) on beraunite
Category Phosphate minerals
(repeating unit)
Ca3(Mn3+)4[(OH)3| (PO4)2]2·3H2O
Strunz classification 8.DH.30
Dana classification
Mitridatite group
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group A2/a
Unit cell a = 17.36 Å, b = 19.53 Å
c = 11.30 Å; β = 96.0°; Z = 12
Color Red, red-brown, deep red, bronzy brown, black
Crystal habit laty to wedge-shaped, pseudorhombohedral crystals; fibrous, in botryoidal to feathery aggregates
Twinning Common perpendicular to {100}
Cleavage Very good {100}
Fracture Micaceous
Mohs scale hardness
Luster Vitreous to resinous or waxy
Streak Chocolate-brown
Diaphaneity Translucent
Specific gravity 3.13 - 3.17
Optical properties Biaxial (-)
Refractive index nα = 1.775 nβ = 1.820 nγ = 1.820
Birefringence δ = 0.045
Pleochroism Strong, X pale red to pink Y,Z deep red brown
2V angle Measured: 8°
References [1][2][3]

Robertsite, Ca3(Mn3+)4[(OH)3| (PO4)2]2·3(H2O) (alternatively formulated Ca2(Mn3(PO4)3O2)(H2O)3), is a secondary phosphate mineral named for Willard Lincoln Roberts (1923–1987), mineralogist and professor at South Dakota School of Mines in Rapid City, South Dakota.

The type locality for Robertsite is the Tip Top mine, Custer County, South Dakota, US. Robertsite occurs at the Tip Top Mine as minute crystal aggregates and crusts found on quartz associated with triphylite. It is a dark reddish brown to black monoclinic mineral.

It occurs as a secondary mineral in pegmatite. It is also reported from the Khoa Rang Kai phosphate deposit, Chiang Mai, Thailand in a limestone guano deposit.[2] It is associated with rockbridgeite, ferrisicklerite, leucophosphite, jahnsite, montgomeryite, collinsite and hureaulite in the type locality. In the guano deposit it occurs with carbonate-fluorapatite, calcite, dolomite, quartz and clay minerals.[2] In the Omo Valley of Ethiopia it occurs with mitridatite associated with fossil fish in Pliocene/Pleistocene lake sediments.[4]

Recently, in an exploration conducted by the Italian La Venta Geographical Association, confirmed the existence of Robertsite in the Puerto Princesa Subterranean River National Park, located about 50 kilometres (31 mi) north of the city center of Puerto Princesa, Palawan, Philippines.[5][6]

  • Mitridatite group:
    • Arseniosiderite-mitridatite series:[7]
      • Ca2(Fe3+)3[(O)2|(AsO4)3]·3H2O
      • to
      • Ca2(Fe3+)3[(O)2|(PO4)3]·3H2O
    • Arseniosiderite-robertsite series:[8]
      • Ca2(Fe3+)3[(O)2|(AsO4)3]·3H2O
      • to
      • Ca3(Mn3+)4[(OH)3|(PO4)2]2·3H2O


  • Andrade, M. B., Morrison, S. M., Di Domizio, A. J., Feinglos, M. N., & Downs, R. T. (2012). Robertsite, Ca2MnIII3O2 (PO4) 3.3 H2O. Acta Crystallographica Section E: Structure Reports Online, 68(10), i74-i75.doi:10.1107/S160053681203735X